Provided are a nitriding apparatus and a method of nitriding, which are capable of suppressing generation of a compound layer by accurately measuring temperature of an object to be treated by nitriding. A nitriding apparatus includes a chamber, a gas supplying unit, a support, a plasma source, a heater, a thermocouple wire including a temperature measuring section, an accommodating member, a power supply for an object to be treated, and a treatment condition control unit. The accommodating member internally accommodates the thermocouple wire to cover the temperature measuring section, while being insulated from the thermocouple wire. The power supply for an object to be treated applies a predetermined voltage to an object to be treated and the housing member so that the object to be treated and the accommodating member are set to an identical potential on the negative side.